1. Technical Field
Embodiments of the present invention relate to a resample ratio error compensation technique for a sample rate conversion algorithm that supports a non-integer rational number multiple resample ratio.
2. Description of the Related Art
In digital signal processing systems, if a radio frequency (RF) band clock has not been synchronized with a baseband clock, a sample rate conversion algorithm is used to change the rate of a signal to a low baseband sampling frequency while maintaining information. In particular, systems supporting various communication standards and frequency bands need to support a non-integer multiple sampling ratio as well as an integer multiple sampling ratio.
As a related sample rate conversion algorithm, there is known an algorithm including L multiple interpolation, a low bandpass filter, and M multiple decimation. Although this sample rate conversion algorithm is the easiest rate conversion method, it is not effective because the complexity of the filter increases if the values L and M are increase.
For this reason, another algorithm that is commonly used is a sample rate conversion method using a fractional delay filter. The fractional delay filter performs correction on a delay equal to or less than 1 clock. The fractional delay filter functions to find and correct the resampling position of output signals when receiving signals sampled at an input sample rate and resampling them at an output sample rate.
Such fractional delay filters chiefly employ a Farrow structure based on a Lagrange polynomial. The reason for this is that fractional delay filters having a Farrow structure are advantageous in that a delay parameter can be controlled because real-time update can be performed and complexity is low compared to a method using a common FIR filter.
                                                        y              ⁡                              (                t                )                                      =                                          ∑                                  i                  =                                      l                    1                                                                    l                  2                                            ⁢                                                          ⁢                                                C                  i                                ⁢                                  x                  ⁡                                      (                    i                    )                                                                                ,                                          ⁢          where                ⁢                                  ⁢                              C            i                    =                                    ∏                                                                                          j                      =                                              I                        2                                                                                                                                                        j                      ≠                      i                                                                                                  l                2                                      ⁢                                                  ⁢                                          t                -                                  t                  j                                                                              t                  i                                -                                  t                  j                                                                                        (        1        )            
Equation 1 represents a Lagrange polynomial and resampling in the form of an equation. In Equation 1, t does not mean a continuous time t, but is a value obtained by representing the position at which resampling needs to be performed as a relative position within 1 symbol period.
Resampling using the fractional delay filter can produce a required sample rate with respect to any input signal, and thus can be effectively used in systems supporting multiple bands because a required rate can be obtained even if an input/output sample rate is flexible.
However, in a sample rate conversion algorithm used for resampling in a structure in which two asynchronous clocks operate at the same time, the resampling using a fractional delay filter causes the deterioration of performance because problems, such as metastability, a phase offset, and a clock rate error, occur.
In order to overcome the problems occurring in asynchronous clocks, a data conversion method using memory was proposed. If the data conversion method using memory is used, the problem of a phase offset can be solved and the problem of metastability can be reduced by safely converting signals using memory without the direct exchange of data between two clock domains upon converting the sample rate of input data.
However, there still remains the problem in which an error occurs between an externally input resample ratio and an actual operating clock rate.
Korean Patent Application Publication No. 1998-0052338 (published on Sep. 25, 1998 and entitled “Apparatus for Correcting Delay Error of Clock Phase Shifter”) discloses an apparatus for correcting the delay error of a clock phase shifter. In particular, there is disclosed the feature of determining the amount of desired phase shift of clocks through the measurement of the amount of shift attributable to a reduction in the delay of a gate and correcting a delay error attributable to voltage, temperature, and processing errors, thereby achieving a stabilized phase shift.
However, the related art does not discloses a feature in which an accurate resampling position is found by compensating for the error of a resample ratio input to a fractional delay filter in a sample rate conversion algorithm that supports a fractional resample ratio and uses asynchronous clocks.